This is a fun introductory geometry lesson for preschoolers and toddlers. It's free in the sense that I am assuming you already have some blocks, but you could collect any number of items around your house to sort into various shapes. Or send your child on a shape scavenger hunt around the house! Idea first published 5/1/09. This post contains affiliate links.
This is my homemade version of a Montessori-style activity using Geometric Solids which matches 3D solids to the corresponding 2D shape. (Note: these solids are much, much less expensive that the official Montessori blue geometric solids.)
Simply divide a large piece of paper into sections labeled with a shape. I showed Kiddo how to match the block to it's corresponding 2D image. A former neighbor gave me a lovely hand-me-down set of natural wooden shapes so at least we have shapes like a cone, pyramid, sphere, half sphere, different prisms, etc. But if you don't have those, look around your home for appropriate items.
The best part of this activity is that is usually ends up in a lot of free block play.
Find more easy diy puzzle ideas for kids along these lines, toddlers to tweens.
Extra Credit reading: